Horner’s syndrome is defined by a constellation of clinical findings, most usually occurring unilaterally, viz.:
- Partial ptosis, due to weakness of Müller’s muscle
- Miosis, due to the unopposed action of the sphincter pupillae muscle, innervated by the parasympathetic nervous system
- Anhidrosis, a loss of sweating (if the lesion is distal to the superior cervical ganglion)
- Enophthalmos, retraction of the eyeball (though this is seldom measured).
The first two mentioned signs are usually the most evident and bring the patient to medical attention; the latter two are usually less evident or absent.
Additional features which may be seen include:
- Heterochromia iridis, different color of the iris (if the lesion is con- genital)
- Elevation of the inferior eyelid due to a weak inferior tarsal mus- cle ("reverse ptosis", or "upside-down ptosis").
Horner’s syndrome results from impairment of ocular sympa- thetic innervation. The sympathetic innervation of the eye consists of a long, three neurone, pathway, extending from the diencephalon down
to the cervicothoracic spinal cord, then back up to the eye via the superior cervical ganglion and the internal carotid artery, and the ophthalmic division of the trigeminal (V) nerve. A wide variety of pathological processes, spread across a large area, may cause a Horner’s syndrome, although many examples remain idiopathic despite intensive investigation. Recognized causes include:
Brainstem/cervical cord disease (vascular, demyelination, syringomyelia)
Malignant cervical lymph nodes
Carotid aneurysm, carotid artery dissection
Involvement of T1 fibers, e.g in T1 radiculopathy, or lower trunk brachial plexopathy
Pearce JMS. Claude Bernard-Horner’s syndrome and Edward Selleck Hare. In: Pearce JMS. Fragments of neurological history. London: Imperial College Press, 2003: 252-255